It is well known that interference between cells is a significant problem in cellular radio access networks. In particular, a base station may suffer from interference on its uplink (that is, it may have difficulty in receiving signals from user equipment devices that have an active connection to the base station, or are trying to establish a connection). In cellular systems, uplink interference generally falls into two main categories: intra-cell interference, caused by user equipment devices transmitting on the same cell; and inter-cell interference, caused by user equipment devices transmitting on adjacent cells. This uplink interference may cause service degradation for the user equipment devices that are connected to the cell that is suffering from the interference. If the base station reacts to this by requiring its connected user equipments to increase their transmit power, these transmissions will in turn cause further uplink interference, which may cause the system to enter an unstable state that will eventually cause the user equipment devices experiencing the weakest links to lose their connection to the system.
This problem is particularly pronounced in frequency reuse-1 systems, where the same frequency is used for transmissions on neighbouring cells.
Intra-cell interference is sometimes referred to as controlled interference, as the affected cell is in charge of allocating the uplink resources and can therefore regulate the uplink interference if necessary. Inter-cell interference is sometimes referred to as uncontrolled interference, as the affected cell is generally not in charge of allocating the uplink resources assigned by adjacent cells.
One possibility to limit uplink interference problems, in Code Division Multiple Access (CDMA) based cellular systems, is the technique of soft handover, whereby a user equipment device may have simultaneous connections to multiple cells, so that each of those cells can control the uplink transmit power of the user equipment device. The base station that experiences the best instantaneous link with the user equipment device will then govern the transmit power. Soft handover reduces the scale of the problem of uplink interference, but in practice it does not eliminate the problem, because the decision as to which of the multiple cells is allowed to control the uplink transmissions of the user equipment is often based on downlink pilot measurements, and these are not necessarily representative of the UL conditions. In any event, using soft handover is not always practical, for example because of capacity issues in one of the affected cells. Implementing soft handover also adds cost, which is undesirable.
Another possibility to limit uplink interference problems, in Orthogonal Frequency Division Multiple Access (OFDMA) based cellular systems, is to apply inter-cell interference control, so that a victim of high uplink interference can notify neighbouring cells about the condition. This enables the aggressor to apply any necessary corrective action, such as altering the way in which it schedules frequency/and or time resources. These techniques, however, are blind techniques, which take no account of user equipment measurements, and rely on blanket actions, such as preventing user equipment devices at the edge of a call from using certain resources. These techniques also assume that there is an established relationship between the victim and the aggressor, in such a way that the victim communicates directly with the aggressor or a set of assumed aggressors, without any further qualification as to whether the recipient is actually contributing towards interference in the victim cell.